A flat light sensor is widely used as a solid-state imaging device, in which photoelectric conversion units (pixels) are two-dimensionally arrayed in a semiconductor, and a signal charge generated by photoelectric conversion in each pixel is transferred and read out by a CCD or a CMOS circuit. Conventional photoelectric units generally have a semiconductor substrate such as a silicon substrate having formed therein a pn junction to provide a photodiode.
With the recent trend to increase the number of pixels, the pixel size has been made smaller, and the area of the individual photodiodes is getting smaller accordingly. This of necessity raises the problem of reduction in the effective area of the photodiode, i.e., reduction of a pixel aperture ratio and reduction in light collection efficiency, resulting in reduction of sensitivity. As a means for improving the aperture ratio and so on a solid state imaging device having an organic photoelectric layer made of an organic material has been under study.
Incorporating a bulk heterojunction structure containing a fullerene or a fullerene derivative to an organic photoelectric layer is known effective to obtain high photoelectric conversion efficiency (high exciton dissociation efficiency). For example, JP 2007-123707A discloses a photoelectric layer containing a fullerene or its derivative.
While the organic photoelectric element used in a solar cell, which is designed to generate electric power, does not need application of an external electric field, the photoelectric element used as a visible light sensor of a solid state imaging device is required to achieve the highest possible photoelectric conversion efficiency and, in some cases, needs application of an external voltage in order to improve photoelectric conversion efficiency or response speed.
In the cases when an external voltage is applied for the purpose of improving photoelectric conversion efficiency or response speed, hole or electron injection from the electrode can occur, which may cause the problem of dark current increase.
Most of the materials generally used as electrodes of a photoelectric element are those having a work function of about 4.5 eV, such as indium tin oxide (ITO). When, for example, fullerene (C60) is used as a material of a photoelectric layer, the energy gap between the work function of the electrode and the LUMO level of the fullerene (C60) is small to allow charge carriers, particularly electrons, to be injected from the electrode into the photoelectric layer. This results in a considerable increase in dark current.
To prevent an increase of dark current due to charge carrier injection, JP 2008-72090A proposes providing a charge blocking layer for efficiently blocking injection of charges into a photoelectric layer thereby to reduce dark current.
However, JP 2007-123707A and JP 2008-72090A are silent to heat resistance, which is an important factor for practical use, giving no concrete description about the structural characteristics of compounds having high heat resistance.
A photoelectric element used in an imaging device must have high heat resistance, given that it is subjected to steps involving heating, such as color filter formation, protective film formation, and soldering, and also from the viewpoint of storage stability.
JP 2005-166637A discloses a triarylamine having a glass transition temperature (hereinafter “Tg”) of 90° C. or higher for use to make an organic electroluminescence device. JP 7-324059A teaches a material having a Tg of around 70° C. for use to make an electrophotographic photoreceptor. However, either JP 2005-166637A or JP 7-324059A gives no mention regarding a photoelectric element.
JP 2005-32852A proposes using a substrate with a Tg of 80° C. or higher in an organic photoelectric element but does not refer to the characteristics of the materials used between electrodes, particularly a photoelectric layer material and a blocking layer material. There is no mention of a method for improving heat resistance of an organic photoelectric element in a very high temperature range, as high as 180° C. or higher, as referred to in the invention.
JP 2006-100508A and JP 2009-200482A discloses a photoelectric element having a photoelectric layer formed by co-deposition of a p type organic photoelectric material and a fullerene without specifically describing the characteristics of the material relevant to heat resistance.